In a noncontact power transmission (noncontact power feed system), it is critically important in ensuring the safety and performing electrical charging to detect a conductor such as a metal, or a circuit including a coil that is present in the vicinity of power transmitting and receiving coils.
Typically, by combined use of a power transmitter and a power receiver, a metallic object that is inserted between the power transmitter (primary-side coil) and the power receiver (secondary-side coil) has been detected based on information on an amplitude and a phase in the event of variation in the load of the power receiver (for example, see Patent Literature 1). Alternatively, a metallic object has been detected from variation in the transmitting and receiving power efficiency (also called the inter-coil efficiency), or from variation in the sensor output by the use of a magnetic sensor, a capacitive sensor, an infrared sensor, or the like.